The present invention relates generally to spindle drive assemblies of the type generally including threaded nut and spindle devices having recirculating balls in load-bearing engagement therebetween to transmit axial forces upon relative rotation of the nut and spindle.
In devices of the type to which the present invention relates, the recirculating balls are guided within a helical ball duct defined between a spindle and a spindle nut, with the balls being continuously recirculated through the helical duct and through ball return means provided on the spindle nut which recirculate the balls to and from the helical duct. The spindle nut, which is usually the outer member, is engaged by an outer clamping device which applies radial compression to the spindle nut in order to insure that the recirculating balls are maintained in load-bearing engagement between the nut and the spindle.
At least one compensating slot may be provided in the spindle nut to effect division of the nut in the circumferential direction. The slot may extend to the helical internal ball guide formed on the spindle nut and defining one side of the helical ball duct, and as a result, the balls will travel past the terminal edges of this compensating slot during their recirculating travel through the assembly.
In a spindle nut of the aforementioned type involving recirculating balls, the diameter of the spindle nut may be reduced by the radial clamping forces of the clamping device until radial play is reduced or eliminated, thereby insuring that axial play between the spindle and the spindle nut is also reduced or eliminated. Moreover, by further radial contracting of the spindle nut it is also possible to generate a prestressing of a selected magnitude between the spindle nut, the balls and the spindle so that, even when higher loads are to be transmitted by the spindle drive of the assembly, which could result in subjecting individual elements of the assembly to a certain degree of deformation, freedom from excessive play will nevertheless be insured.
An example of a prior art device of the type discussed may be found in U.S. Pat. No. 888,619. In the device of this patent, the nut is formed in longitudinal sections which are divided circumferentially of the device. This division is generally made for reasons of machining, for example, to enable machining or finishing of a ball return duct of such a device. However, in the assembly of a spindle drive, the two halves of the spindle nut are joined together completely without play so that no slot in the sense of the slot previously mentioned in connection with the present invention is formed and as a result, there will arise problems which result from jolts or shocks when the balls travel past the joint, thereby causing irregular or rough operation of the spindle assembly.
In another prior art device of the type discussed above, the spindle nut is also divided in a longitudinal plane and the resulting halves of the spindle nut remain connected through an elastic web arranged outside of the threaded bore. An example of this type of device may be found in U.S. Pat. No. 2,694,942. The slot which is intersected twice by all the threads of the spindle nut at diametrically oppositely located sides is of a relatively large width. As a result, the spindle nut can be contracted by suitable means to such an extent that the internal threads will assume an elliptical shape. Because of this, in the region of the minor axis of the ellipse, there will be a loss of play between the balls and the threads of the spindle and the spindle nut, while in the region of the major axis of the ellipse the balls will have a distinct play in the circumferential direction and thus also in the axial direction, whereby the balls may not operate to transmit the desired forces through the assembly. The purpose of this measure is to achieve a transfer of the balls without jolting into and out of the return duct by arranging the inlet and the outlet of the return duct in the non-load bearing region of the spindle nut. Since the slots are also arranged in the non-load bearing region of the spindle nut, there will be generally eliminated problems resulting from jolts occurring in spite of the large width of the slots. However, in a prior art device of this type, there arises a significant disadvantage in that only a portion of the balls is utilized for transmitting the forces between the spindle and spindle nut so that contact pressures and wear are significantly increased. Additionally, in the case where predetermined contact pressures are maintained, the spindle drive, and particularly the spindle nut, will have much larger dimensions.
The present invention is directed to providing an improved spindle drive assembly wherein the recirculating balls may move through the helical ball duct of the assembly past the terminal edges of a slot or slots formed in the spindle nut without jolting or rough, uneven operation despite the fact that the balls are maintained under load-bearing condition. Furthermore, the invention seeks to achieve these operating characteristics without causing increased contact pressure due to incomplete utilization of the number of recirculating balls of the assembly and without increasing the structural dimensions of the device.